This invention relates broadly to a method for more efficiently utilizing heat in furnace operations. More particularly, it relates to a method for substantially increasing the emissivity and luminosity of furnace hot gases so that they become better radiators of heat to the furnace charge. This permits heat which otherwise would leave the furnace with the exhaust gases to be utilized in the furnace, thus substantially increasing the economies of furnace operation. The invention, although described herein in terms of a reverberatory furnace, finds utility in any application where a substance is being at least partly heated by radiation from a gas which is at a higher temperature than the substance.
Furnaces and their efficient and economical operation are an important aspect of many commercial manufacturing processes. They are widely used, for example, in the pyrometallurgical processing of mineral ores. The reverberatory and open-hearth furnaces used in the production of copper and steel are but two well-known examples of important industrial furnaces. These furnaces normally operate at very high temperatures, e.g., 1000.degree. to 3000.degree. F., depending upon such factors as their particular function and the nature of the material being processed. They are huge consumers of energy in the form of the fuel which is required to fire such furnaces.
In many furnaces, the heat is supplied by burning a fuel in proximity to a bed of the substance which is being treated in the furnace. This substance is often referred to as the "furnace charge" or "charge." This burning normally produces a highly visible flame in a part of the furnace generally referred to as the "combustion zone," and hot combustion gases throughout the rest of the furnace. The charge is heated in large part by the heat transferred to it by radiation from the flame and hot combustion gases.
The method of this invention and the problems it is intended to overcome are conveniently demonstrated by consideration of a conventional industrial reverberatory furnace of the type used, for example, to process and melt copper ores. A typical reverberatory furnace of this type is shown in FIG. 1.
The furnace 10 contains an elongated refractory-lined chamber 11 having a relatively high length to height ratio. The ore 12, for example a copper concentrate, is fed to the furnace through the refractory roof 13 from feed hoppers 13a and deposited on the furnace hearth 14 between the banks 14a (see FIG. 3) on each side of the furnace. FIG. 1, for reasons of clarity, shows burners 15 at only one end of the furnace. Those skilled in the art will appreciate, however, that in many such furnaces, there are burners at each end for cyclic firing of the furnace, first from one end and then from the other.
An oxidant 16, usually air, and a fuel 17, such as natural gas, oil or various other energy sources, are fed to the burners 15 where they are mixed in desired proportions. The fueloxidant mixture is injected into the furnace interior 11 through ports 18 located approximately midway in the furnace end wall 19. As the mixture enters the furnace, it ignites to produce a distinct visible flame 20 at one end of the furnace. This flame normally extends only part way down the length of the furnace and generally delineates the zone of combustion. Enough burners are usually provided to maintain a substantially continuous flame front across the width of the charge 22.
In conventional furnace operation, flame 20 can extend from about one-fifth to one-third of the way down the furnace length, as shown in FIG. 1. The disappearance of flame 20 part-way down the furnace indicates that fuel combustion has been substantially completed at that point. The remaining portion of the furnace downstream of flame 20, generally designated as 24, is filled with hot furnace combustion gases which normally are relatively invisible or transparent as compared to the flame 20. These gases pass through the furnace over the charge as generally designated by the arrows in FIG. 1. Typical temperature present in a reverberatory furnace of this type range from about 2100.degree. to 2700.degree. F., and typically from about 2200.degree. to 2600.degree. F.
The flame 20 and hot combustion gases radiate heat to the charge 22. This heat, plus that radiated from the furnace roof 13, converts the charge 22 (i.e., the fresh ore 12) to the molten state. The ore then separates into a heavier matte layer 25 enriched in copper values, and a lighter slag layer 26 depleted in copper values. These layers are separated in conventional fashion by "tapping" the furnace.
The hot exhaust gases 27 exit the furnace through an outlet 30. Unfortunately, the heat radiation characteristics of the hot furnace gases leave much to be desired. As a result, only a fraction of the heat values of these gases is radiated to the charge 22 in the zone 24 downstream of flame 20. The remaining heat values are retained in the hot gas and leave the furnace with the exhaust gases 27. In many operations, the heat present in the exhaust gases is so significant, e.g., as much as 30 to 40% of the heating value of the fuel 17, that it becomes economically necessary to recover it. This can be done by passing the hot exhaust gases 27 through "checkers," waste heat boilers, or other heat regeneration means (not shown in FIG. 1).
It is apparent, therefore, that heat utilization within the furnace itself is quite inefficient. This problem has recently become of even more serious concern because of the drastically increased cost of energy, and the potential future shortage of energy sources.
A major problem confronting those who would improve the thermal efficiency of such furnaces is how to make the hot furnace gases "give up" more of their heat by radiation to the furnace charge as they pass through the furnace.
The mathematical equations and variables which effect the rate of heat transfer from a hot substance (the furnace gases) to a cooler one (the furnace charge) by a radiation mechanism have been known for years. See, for example, Industrial Furnaces by Trinks and Mawhinney, Volume 1, 5th Edition, Wiley and Sons (1961), pages 44 and 50 (hereinafter "Trinks and Mawhinney"). For example, the amount of radiant heat transfer in a furnace can be increased by increasing the temperature of the furnace gases since the amount of heat transferred by radiation is known to be directly proportional to the value of: EQU (T.sub.g).sup.4 - (T.sub.c).sup.4
where T.sub.g is the temperature of the hot gases and T.sub.c is the temperature of the charge. Normally, however, an increase in hot gas temperature would only necessitate greater instead of lesser fuel consumption. Furthermore, the operating temperatures of many industrial furnaces are normally relatively fixed by factors such as the nature of the material being processed, the materials of construction used in the furnace, and the like.
It is also known that the amount of heat radiated from the hot gases to the charge can be increased by increasing the "emissivity" of the hot gases. The emissivity of a gas is a measure of the rate at which the gas radiates heat in relation to an arbitrarily defined norm or standard for perfect or complete heat radiation called a "black body." Each gas can be assigned a numerical "emissivity factor" which can vary from a value of almost zero to slightly less than 1.0. A value of about zero means that the gas radiates virtually no heat while a value of about 1.0 means it radiates heat at about the same rate as a black body. Thus, as the value of the "emissivity factor" of a gas increases, the amount of radiant heat transfer from the gas also increases. A desirable objective then would be to increase the emissivity of the hot furnace gases. Those skilled in the art often refer to flames or gases of high emissivity as being "luminous" and those of low emissivity as being "non-luminous." In general, a "luminous" gas in the context of furnace operation is one that is opaque to varying degrees, while a "non-luminous" gas is one which is essentially clear or transparent. The properties of emissivity and luminosity are often used interchangeably in the prior art, apparently because of the general belief that the luminosity of a gas is usually directly proportional to its emissivity.
A generally recognized separation point between gases of low and high emissivity occurs at an emissivity factor of about 0.3. For example, furnace hot gases whose emissivity factors are below about 0.3 are generally regarded as being of low emissivity or non-luminous, while those whose emissivity factors are above about 0.3 are generally regarded as being of high emissivity or luminous. For practical purposes, emissivity factors of above about 0.6 are preferred.
There are many known techniques for increasing the emissivity or luminosity of a gas. See, for example, The Science of Flames and Furnaces by M. W. Thring, 2nd Edition, Wiley and Sons (1968), particularly pp. 314-357 (hereinafter "Thring"). For example, the addition of finely divided solid particles to a gas will normally increase its emissivity and radiant heat transfer (see Thring, page 321). These particles can be generated in a number of ways, including the incomplete combustion and cracking of hydrocarbons (see Thring, page 322). U.S. Pat. No. 2,126,724 teaches at column 1, lines 11-27, that radiant heat transfer in a furnace is greatly promoted by a luminous burner flame, and that such luminosity can be provided by mixing minute particles of carbon with the fuel fed to the furnace. The particles are heated to incandescence in the burner flame and give the flame its luminous appearance. However, one problem with this approach is that the cooling effect of the heat capacity of the particles is significant and could lower the flame temperature to the point where any increased radiation resulting from the particles is more than offset by the reduction in radiation produced by the lower flame temperature (see Thring, page 321).
In the case of certain furnace fuels, flame emissivity and luminosity can be increased by altering the fuel-oxidant ratio. See, for example, the discussion by Trinks and Mawhenney at page 44, where it is pointed out that a burner flame of higher emissivity is produced when the fuel-oxidant mixture contains excess natural gas as opposed to excess air. These conditions crack the hydrocarbons in the fuel to carbon particles which increase the emissivity of the burner flame. However, as the fuel is combusted, eventually no new carbon particles are produced while those that have been produced are gradually burned away. Finally a point is reached where all the carbon particles have been burned at which time the flame extinguishes. The emissivity then drops sharply. Since the burner flame extends only part-way down the furnace, the region of high emissivity exists in only a short section of the furnace.
The emissivity of the burner flame is also known to depend on the chemical nature of the fuel used. For example, Thring points out at pages 337-338 the significant effect of the carbon to hydrogen weight ratio of the fuel upon the emissivity of the flame produced by the combustion of the fuel.
While the above general principles are helpful, problems have arisen in usefully applying them to large industrial furnaces. Numerous attempts have been made to provide within the furnace hot gases an environment of finely sized particles in the hope of improving the radiant heat transfer from the gases to the charge. For example, U.S. Pat. Nos. 2,126,724, 2,298,842 and 3,345,054 disclose techniques for mixing finely divided carbon particles with the furnace fuel before it enters the furnace. The carbon particles are provided by thermally cracking a system of hydrocarbon gas in a cracking chamber outside the furnace, and then mixing the outlet stream from the cracking chamber (which includes the carbon particles) with the furnace fuel before it enters the furnace. The objective is to provide carbon particles in the burner flame so it will be more emissive and luminous, thereby radiation more of its heat to the charge.
There are several drawbacks to mixing the carbon particles with the burner fuel. First, a separate hydrocarbon cracking system separate from the furnace and complete with its own cracking vessel, etc., is required. Secondly, finely sized carbon particles have a tendency to agglomerate. Larger size particles do not enhance flame emissivity to the extent that smaller particles do. Since the carbon particles are generated outside the furnace, there is a delay between their generation and introduction into the furnace, during which particle agglomeration could occur. Finally, and probably most important, the carbon particles will at most only increase the emissivity and luminosity of the burner flame 20 (see FIG. 1). As pointed out above, this flame extends at most only about one-third the way down the furnace. Since the carbon particles will be consumed or burned up in the flame (see Trinks and Mawhenney, page 44), substantially none or very few will carry into the remaining two-thirds of the furnace (zone 24 in FIG. 1) where a substantial amount of the charge heating takes place. Therefore, the emissivity and luminosity of the hot furnace gases downstream of the flame will not be increased by the injection of carbon particles. Since these downstream hot gases do a lion's share of heating the furnace charge, the limitations of mixing carbon particles with the fuel become evident.
These limitations are appreciated by those skilled in the art. As Thring states on page 347: "Unfortunately, there is at present no known way of producing flames where the emissivity is maintained high throughout the length of the furnace while combustion is completed in the first third." Thring goes on to note later on page 347: "There is, however, clear scope for improvement if a method can be found for maintaining the emissivity of a flame at a fairly high level even though combustion is effectively complete, e.g., by producing soot particles which do not disappear for some time after the remainder of the fuel is fully burned out."
It is thus the inability to maintain the emissivity of the furnace hot gases at a high level throughout the portions of the furnace downstream of the flame or combustion zone which has posed a major obstacle to the more efficient extraction and utilization of the heat value of the hot gases while still within the furnace. It is toward the solution of this problem that the present invention is directed.
It is, therefore, an object of this invention to provide a method to increase the emissivity of furnace hot gases, and consequently their radiant heat transfer to the furnace charge, and to maintain the increased emissivity throughout the furnace environment.
It is another object of this invention to provide a method to increase the emissivity of furnace hot gases in the region of the furnace downstream of the burner flame.
It is another object of this invention to provide a method to selectively increase the emissivity of the furnace hot gases at one or more different locations in the furnace, as desired, including locations downstream of the burner flame.
It is another object of this invention to provide a method to increase the emissivity of furnace hot gases by providing and maintaining an atmosphere of carbon particles in the gases downstream of the burner flame.
It is another object of this invention to provide a method to increase the emissivity of furnace hot gases by providing carbon particles therein, but where the particles originate from a source other than the fuel injected into the furnace.
It is another object of this invention to provide a method to increase the emissivity of furnace hot gases by generating carbon particles within the furnace, downstream of the combustion zone of the furnace.
It is another object of this invention to provide a method to increase the emissivity of furnace hot gases by providing carbon particles therein while at the same time adding heat to the hot gases.
It is a further object of this invention to provide a method to increase the emissivity of furnace hot gases by providing carbon particles therein without substantially decreasing the temperature of the hot gases because of heat transfer from the hot gases to the particles.
It is a general object of this invention to provide a method for increasing the amount of heat radiated by furnace hot gases to a furnace charge.
It is another general object of this invention to provide a method to improve the thermal efficiency of the radiant heat transfer in a furnace in which the charge is at least partly heated by radiation from hot gases.
It is another general object of this invention to provide a method of furnace operation whereby the quantity of furnace fuel consumed per unit of charge is decreased, thereby permitting the economies of lower fuel consumption or increased furnace through-put at no added cost.
These and other objects of the invention will be apparent to those skilled in the art from a consideration of this entire specification and the accompanying drawings.